EP1117484B1 - Multilayered shell catalysts for catalytic gaseous phase oxidation of aromatic hydrocarbons - Google Patents

Abstract

A coated catalyst for the catalytic gas-phase oxidation of aromatic hydrocarbons comprises, on an inert nonporous support, a catalytically active composition comprising a defined amount of vanadium oxide, titanium dioxide, a cesium compound, a phosphorus compound, and antimony oxide, wherein the catalytically active composition is applied in two or more layers and where relative to the inner layer or inner layers the outer layer has an antimony oxide content which is from 50 to 100% lower and wherein the amount of catalytically active composition of the inner layer or layers is from 10 to 90% by weight of the amount of catalytically active composition, and can be used for preparing carboxylic acids and/or anhydrides, in particular phthalic anhydride; also specified is a production process for such catalysts.

Description

The invention relates to a coated catalyst for the catalytic gas-phase oxidation of aromatic hydrocarbons, containing a catalytically active composition with a content, based on the total amount of the catalytically active composition, of 1 to 40% by weight of vanadium oxide, calculated on an inert non-porous support as V ₂ O _5, 60 to 99% by weight of titanium oxide, calculated as TiO ₂ , up to 1% by weight of a cesium compound, calculated as Cs, up to 1% by weight of a phosphorus compound, calculated as P and up to a total content of 10% by weight of antimony oxide, calculated as Sb ₂ O ₃
The invention further relates to a production process for these catalysts and a process for the production of carboxylic acids and / or carboxylic anhydrides and in particular phthalic anhydride using these catalysts.

It is known that a large number of carboxylic acids and / or Carboxylic anhydrides technically through the catalytic gas phase oxidation of aromatic hydrocarbons such as benzene Xylenes, naphthalene, toluene or durol, in fixed bed reactors, preferably tube bundle reactors. For example Benzoic acid, maleic anhydride, phthalic anhydride, Isophthalic acid, terephthalic acid or pyromellitic anhydride won. This is generally done using a mixture of one molecular oxygen-containing gas, for example air, and the starting material to be oxidized by a variety in one Reactor arranged pipes passed in which there is a bed at least one catalyst. For temperature control are the tubes of a heat transfer medium, for example one Melted salt, surrounded. Despite this thermostatting, it can get in the catalyst bed to form so-called "hot spots" (hot spots) come in which there is a higher temperature than in the rest of the catalyst bed. These "hot spots" give rise to side reactions, such as the total combustion of the Starting material or they lead to the formation of undesirable, from The reaction product cannot be separated or can only be separated with a lot of effort By-products, for example for the formation of phthalide or Benzoic acid, in the manufacture of phthalic anhydride (PSA) from o-xylene. Furthermore, the formation of a pronounced prevents hot spots a rapid startup of the reactor on the Reaction temperature of the implementation, as from a certain hot spot temperature the catalyst can be irreversibly damaged can, so the load increase only in small increments is feasible and must be checked very carefully.

In order to weaken this hot spot, the technology has started to differently active catalysts in layers to arrange the catalyst bed, usually the less active catalyst is arranged in the fixed bed so that the reaction gas mixture comes into contact with it first, i.e. it lies in the bed towards the gas inlet, whereas the more active catalyst for gas escape from the catalyst bed is located. The different active catalysts in the Catalyst bed can the reaction gas at the same temperature exposed, the two layers can be made differently active catalysts but also on different Reaction temperatures thermostatted with the reaction gas be brought into contact (DE-A 40 13 051).

So-called shell catalysts have proven themselves as catalysts for these oxidation reactions, in which the catalytically active mass is in the form of a shell on a carrier material which is generally inert under the reaction conditions, such as quartz (SiO ₂ ), porcelain, magnesium oxide, tin dioxide, silicon carbide, rutile, alumina (Al ₂ O ₃ ), aluminum silicate, magnesium silicate (steatite), zirconium silicate or cerium silicate or mixtures of these carrier materials is applied. In addition to titanium dioxide in the form of its anatase modification, vanadium pentoxide is generally used as the catalytically active component of the catalytically active composition of these coated catalysts. In addition, the catalytically active composition can contain a large number of other oxidic compounds in small amounts which, as promoters, influence the activity and selectivity of the catalyst, for example by lowering or increasing its activity. Examples of such promoters are the alkali metal oxides, in particular lithium, potassium, rubidium and cesium oxide, thallium (I) oxide, aluminum oxide, zirconium oxide, iron oxide, nickel oxide, cobalt oxide, manganese oxide, tin oxide, silver oxide, copper oxide, chromium oxide, molybdenum oxide, Tungsten oxide, iridium oxide, tantalum oxide, niobium oxide, arsenic oxide, antimony oxide, cerium oxide and phosphorus pentoxide called. The alkali metal oxides, for example, act as promoters which reduce the activity and increase the selectivity, whereas oxidic phosphorus compounds, in particular phosphorus pentoxide, increase the activity of the catalyst but reduce its selectivity.

For the production of such coated catalysts according to the Process of DE-A 16 42 938 and DE-A 17 69 998 an aqueous and / or a solution containing an organic solvent or Suspension of the active mass components and / or their precursor compounds, which is referred to below as "mash", onto the carrier material in a heated coating drum sprayed at an elevated temperature until the desired proportion of active mass of total catalyst weight is reached. According to DE 21 06 796 the coating can also be carried out in fluidized coaters, such as are specified in DE 12 80 756. At the Spray on in the coating drum and when coating in However, fluidized bed losses are high because of considerable losses Amounts of the mash are atomized or by parts of the abrasion already coated active composition rubbed again and by the Exhaust gas are discharged. Because the active mass share of the total catalyst generally only a small deviation from the setpoint should have, because of the amount of the applied active mass and the layer thickness of the shell activity and selectivity of the catalyst are strongly influenced, the catalyst in the described Manufacturing methods often used to determine the applied Active mass quantity cooled, from the coating drum or removed from the fluidized bed and weighed. Will be too much Active mass deposited on the catalyst support is generally a subsequent, gentle removal of too much applied Active mass quantity without impairing the strength the shell, especially without cracking in the catalyst shell, not possible.

To alleviate these problems, technology has been adopted to the mash organic binder, preferably copolymers, advantageous in the form of an aqueous dispersion of vinyl acetate / vinyl laurate, Vinyl acetate / acrylate, styrene / acrylate, vinyl acetate / maleate and also add vinyl acetate / ethylene, with binder amounts of 10-20% by weight, based on the solids content of the mash, were used (EP-A 744 214). Will the mash be without organic binders applied to the carrier, so are coating temperatures an advantage over 150 ° C. With addition above The specified coating temperatures are the usable coating temperatures depending on the binder used between 50 and 450 ° C (DE 21 06 796). The applied binders burn after the Filling the catalyst into the reactor and starting up the Reactor within a short time. The binder additive also has the advantage that the active mass adheres well to the carrier, so that the transportation and filling of the catalyst are made easier.

Gas phase oxidation on the shell catalysts described above do not find only on the outer surface of the bowl instead of. To achieve a full implementation of the in technical processes applied high loads of the reaction gas catalyst activity and selectivity required with starting material is an efficient use of the entire active mass shell of the catalyst and thus easy access to the reaction gas to the reaction centers in this bowl necessary. Because the oxidation of aromatic compounds to carboxylic acids and / or carboxylic anhydrides over a variety of Intermediate stages and the product of value on the catalyst can be further oxidized to carbon dioxide and water Achieving a high turnover of educt while pushing it back optimal oxidative degradation of the valuable product Adjustment of the residence time of the reaction gas in the active mass by creating a suitable active mass structure (for example their porosity and pore radius distribution) in the Catalytic converter shell required.

It should also be borne in mind that the gas composition not necessarily on the outer surface of the active material shell also the gas composition at inner points of the active mass equivalent. Rather, it is expected that concentration higher in primary oxidation products, the educt concentration in contrast, is lower than on the outer catalyst surface. This different gas composition should through a targeted adjustment of the active mass composition within the active mass shell to be taken into account to achieve optimal catalyst activity and selectivity. DE 22 12 964 already has a sequential method Spraying on mashes of various compositions and the use of the catalysts thus obtained for the production described by phthalic anhydride.

The multilayer coated catalysts thus obtained result but still not satisfactory results and have the disadvantage that when used to oxidize o-xylene only unsatisfactory Yields of phthalic anhydride can be achieved.

The object was therefore to produce multilayer coated catalysts which allow a further increase in the selectivity of the oxidation of aromatic hydrocarbons to carboxylic acids.
This object was achieved by a coated catalyst for the catalytic gas-phase oxidation of aromatic hydrocarbons, containing a catalytically active composition with a content, based on the total amount of the catalytically active composition, of 1 to 40% by weight of vanadium oxide on an inert non-porous support , calculated as V ₂ O ₅ , 60 to 99% by weight of titanium oxide, calculated as TiO ₂ , up to 1% by weight of a cesium compound, calculated as Cs, up to 1% by weight of a phosphorus compound, calculated as P and up to a total content of 10% by weight of antimony oxide, calculated as Sb ₂ O ₃ , which is characterized in that the catalytically active composition is applied in two or more than two layers, the inner layer or the inner layers having an antimony oxide content of 1 to 15 wt .-% and the outer layer have a 50 to 100% reduced antimony oxide content and the amount of the catalytically active mass of the inner ski or the inner layers is 10 to 90% by weight of the amount of the total catalytically active composition.

Furthermore, a manufacturing process for these catalysts and a process for the production of carboxylic acids and / or carboxylic anhydrides and in particular of phthalic anhydride Found use of these catalysts.

The layer thickness of the inner layer or the sum of the layer thicknesses the inner layers are usually 0.02 to 0.2 mm, preferably 0.05 to 0.1 mm and the outer layer 0.02 to 0.2 mm, preferably 0.05 to 0.1 mm.

The new catalysts preferably contain two concentric ones Layers of catalytically active mass, the inner layer preferably 2 to 10 and in particular 5 to 10% by weight of vanadium oxide and preferably 2 to 7, in particular 2.5 to 5% by weight of antimony oxide and the outer layer preferably 1 to 5, in particular 2 to 4 % By weight of vanadium oxide and preferably 0 to 2, in particular 0 to 1 Contains wt .-% antimony oxide.

In addition, the coated catalysts contain further constituents known per se for the oxidation of aromatic hydrocarbons to carboxylic acids, such as titanium dioxide in the anatase form with a BET surface area of 5 to 50 m ² / g, preferably 13 to 28 m ² / g.

The non-porous carrier made of inert material consists, for. B. from quartz (SiO ₂ ), porcelain, magnesium oxide, tin dioxide, silicon carbide, rutile, alumina (Al ₂ O ₃ ), aluminum silicate, magnesium silicate (steatite), zirconium silicate or cerium silicate or mixtures of these carrier materials. Steatite in the form of balls with a diameter of 3 to 6 mm or rings with an outer diameter of 5 to 9 mm and a length of 4 to 7 mm is preferably used.

In addition to the optional additives cesium and In principle, phosphorus can be found in the catalytically active mass small amounts of a variety of other oxidic compounds be included as promoters of activity and selectivity of the catalyst, for example, by its Reduce or increase activity. As such are promoters examples include the alkali metal oxides, especially in addition to the above Cesium oxide, lithium, potassium and rubidium oxide, Thallium (I) oxide, aluminum oxide, zirconium oxide, iron oxide, nickel oxide, Cobalt oxide, manganese oxide, tin oxide, silver oxide, copper oxide, Chromium oxide, molybdenum oxide, tungsten oxide, iridium oxide, tantalum oxide, Niobium oxide, arsenic oxide, antimony oxide, cerium oxide called. Usually however, cesium from this group is used as a promoter. In addition, the promoters mentioned also preferably come as additives the oxides of niobium, tungsten and lead in amounts from 0.01 to 0.50 wt .-%, based on the catalytically active mass, into consideration. As increasing the activity but reducing the selectivity Additive are mainly oxidic phosphorus compounds, especially phosphorus pentoxide.

This is usually the inner layer of the catalyst containing phosphorus and the outer layer low in phosphorus or free of phosphorus.

(a) Aufsprühen von Lösungen oder Suspensionen in der Dragiertrommel,

(b) Beschichtung mit einer Lösung oder Suspension in einer Wirbelschicht oder

(c) Pulverbeschichtung der Träger

The application of the individual layers of the coated catalyst to the inert non-porous support can be done by any known methods, e.g. B. by

(a) spraying on solutions or suspensions in the coating drum,

(b) coating with a solution or suspension in a fluidized bed or

(c) powder coating the supports

To a)

Sequential spraying is generally done as described in DE 22 12 964 and EP 21325, with the proviso that if possible chromatography effects, d. H. the emigration of individual components in the other layer can be avoided should. If the active components to be applied are not at least can sometimes exist as insoluble metal compounds it may be expedient to apply the thermal powder Undergo pretreatment, or in another way e.g. through additives to make them practically insoluble.

To b)

The coating in the fluidized bed can be as described in DE 12 80 756 respectively.

To c)

The method of powder coating, which from WO-A 98/37967 and EP-A 714 700 is also known for sequential coating can be applied in several layers. To do this, the Solution and / or suspension of the catalytically active metal oxides, if necessary with the addition of auxiliaries, initially powder produced, one after the other, if necessary with an intermediate Heat treatment, shell-shaped applied to the carrier become.

To remove volatile components, the catalyst is in usually, at least subsequently, subjected to a heat treatment.

The new catalysts are generally suitable for the gas phase oxidation of aromatic C ₆ to C ₁₀ hydrocarbons, such as benzene, the xylenes, toluene, naphthalene or durol (1,2,4,5-tetramethylbenzene) to carboxylic acids and / or carboxylic acid anhydrides such as maleic anhydride, phthalic anhydride , Benzoic acid and / or pyromellitic dianhyride.

In particular, the new shell catalysts make it possible significant increase in selectivity and yield in the Manufacture of phthalic anhydride.

For this purpose, the catalysts prepared according to the invention are filled into reaction tubes thermostatted from the outside to the reaction temperature, for example by means of molten salt, and the reaction gas is prepared at the catalyst bed prepared in this way at temperatures of generally 300 to 450, preferably 320 to 420 and particularly preferably 340 to 420 400 ° C and at an overpressure of generally 0.1 to 2.5, preferably 0.3 to 1.5 bar with a space velocity of generally 750 to 5000 h ^{-1 passed} .

The reaction gas supplied to the catalyst is generally produced by mixing a gas containing molecular oxygen, which in addition to oxygen may also contain suitable reaction moderators and / or diluents, such as steam, carbon dioxide and / or nitrogen, with the aromatic hydrocarbon to be oxidized, where the gas containing the molecular oxygen generally 1 to 100, preferably 2 to 50 and particularly preferably 10 to 30 mol% oxygen, 0 to 30, preferably 0 to 10 mol% water vapor and 0 to 50, preferably 0 to 1 mol% % Carbon dioxide, balance nitrogen. To generate the reaction gas, the molecular oxygen-containing gas is generally charged with 30 g to 150 g per Nm ^{3 of} gas of the aromatic hydrocarbon to be oxidized.

The gas phase oxidation is advantageously carried out in such a way that two or more zones, preferably two zones in the reaction tube located catalyst bed on different Reaction temperatures thermostatted, for example reactors with separate salt baths, as described in DE-A 22 01 528 or DE-A 28 30 765 are described, can be used. Becomes the reaction is carried out in two reaction zones, as in DE-A 40 13 051, the gas entry of the Reaction gas located reaction zone, which in general 30 to 80 mol% of the total catalyst volume comprises, to a by 1 to 20, preferably by 1 to 10 and in particular by 2 to 8 ° C higher reaction temperature than that at the gas outlet Reaction zone thermostated. Alternatively, the gas phase oxidation even without division into temperature zones for a single one Reaction temperature are carried out. Independent of Temperature structuring has proven to be particularly advantageous proven if in the above reaction zones Catalyst bed catalysts are used which are in their catalytic activity and / or chemical composition distinguish their active mass. Preferably when used two reaction zones in the first, i.e. for the gas entry of the reaction gas reaction zone located, a catalyst used, the compared to the catalyst, which is in the second reaction zone, i.e. towards the gas outlet, has a slightly lower catalytic activity. In general the implementation is controlled by the temperature setting so that in the first zone most of that in the reaction gas contained aromatic hydrocarbon at maximum Yield is implemented.

Is the PSA production with the catalysts of the invention using several reaction zones in which different Catalysts are carried out, so can the new coated catalysts were used in all reaction zones become. However, there can already be significant benefits in general compared to conventional methods, if only in one of the reaction zones of the catalyst bed, for example the first reaction zone, a coated catalyst according to the invention is used and in the other reaction zones, for example the second or last reaction zone, to conventional ones Shell catalysts produced in this way can be used.

Examples

The anatase used contained: 0.18% by weight S, 0.08% by weight P, 0.24% by weight of Nb, 0.01% by weight of Na, 0.01% by weight of K, 0.004% by weight of Zr, 0.004 wt% Pb.

Example 1: Preparation of the coated catalyst Ia - comparison

From a suspension consisting of 250.0 g of anatase, which had a BET surface area of 20 m ² / g, 13.6 g of vanadyl oxalate (= 7.98 g of V ₂ O ₅ ), 1.37 g of cesium sulfate (= 1 , 01 g Cs), 940 g water and 122 g formamide were in a spray dryer at a gas inlet temperature of 280 ° C and a gas outlet temperature of the drying gas (air) of 120 ° C 270 g powder with a particle size of 3 to 60 microns for 90 wt .-% of the powder produced. After calcination of the powder (1 h at 400 ° C.), 90 g of the calcined powder were mixed with 10 g of melamine. 700 g of steatite (magnesium silicate) rings with an outer diameter of 8 mm, a length of 6 mm and a wall thickness of 1.5 mm were added in a coating drum at 20 ° C. for 20 minutes with 93 g of the powder mixed with melamine coated by 56 g of a 30 wt .-% water / 70 wt .-% glycerol mixture. The catalyst support coated in this way was then dried at 25 ° C. The weight of the catalytically active composition applied in this way was 10.7% by weight, based on the total weight of the finished catalyst, after heat treatment at 400 ° C. for 1/2 h. The applied, catalytically active mass, that is to say the catalyst shell, consisted of 0.40% by weight of cesium (calculated as Cs), 3.0% by weight of vanadium (calculated as V ₂ O ₅ ) and 96.6% by weight. % Titanium dioxide.

Example 2: Preparation of the coated catalyst Ib - comparison

The procedure was as described in Example 1, but using a suspension consisting of 400.0 g of anatase, which had a BET surface area of 21 m ² / g, 57.6 g of vanadyl oxalate (= 33.8 g of V ₂ O ₅ ) , 2.75 g of cesium sulfate (= 2.02 g of Cs), 14.4 g of antimony trioxide, 2.5 g of ammonium dihydrogen phosphate (= 0.67 g of P), 1500 g of water and 196 g of formamide. The applied catalytically active composition consisted of 0.15% by weight of phosphorus (calculated as P), 7.5% by weight of vanadium (calculated as V ₂ O ₅ ), 3.2% by weight of antimony (calculated as Sb ₂ O ₃ ), 0.45% by weight of cesium (calculated as Cs) and 89.05% by weight of titanium dioxide.

Example 3: Preparation of the coated catalyst Ic comparison

The procedure was as described in Examples 1 and 2 with the modification that that initially 46 g of the powder described in 1 and then 47 g of the powder described in 2 on the Carrier were applied.

Example 4: Preparation of the shell catalyst Id comparison

700 g of steatite (magnesium silicate) rings with an outer diameter of 8 mm, a length of 6 mm and a wall thickness of 1.5 mm were heated in a coating drum to 160 ° C. and with a suspension of 400.0 g of anatase with a BET -Surface area of 20 m ² / g, 57.6 g of vanadyl oxalate (= 33.8 g of V ₂ O ₅ ), 14.4 g of antimony trioxide, 2.5 g of ammonium dihydrogen phosphate (= 0.67 g of P), 2.44 g Sprayed cesium sulfate (= 1.79 g Cs), 618 g water and 128 g formamide until the weight of the layer applied was 10.5% of the total weight of the finished catalyst. The catalytically active composition applied in this way, i.e. the catalyst shell, consisted on average of 0.15% by weight of phosphorus (calculated as P), 7.5% by weight of vanadium (calculated as V ₂ O ₅ ), 3, 2% by weight of antimony (calculated as Sb ₂ O ₃ ), 0.4% by weight of cesium (calculated as Cs) and 89.05% by weight of titanium dioxide.

Example 5: Preparation of the catalyst IIa - according to the invention

The procedure was as described in Example 3 with the proviso that initially 46 g of the powder described in Example 2 and then 47 g of the powder described in Example 1 on the Carrier were applied.

Example 6: Preparation of Catalyst IIb - According to the Invention

The procedure was as described in Example 5 with the modification that that the powder according to Example 2.61.5 g instead of 57.6 g vanadyl oxalate contained.

Example 7: Preparation of the catalyst IIc - according to the invention

The procedure was as described in Example 5 with the modification that that the powder according to Example 2 20.02 g instead of 14.4 g antimony trioxide contained.

Example 8: Preparation of the catalyst IId - according to the invention

The procedure was as described in Example 5 with the modification that that the powder according to Example 2 9.0 g instead of 14.4 g of antimony oxide contained.

Example 9: Preparation of Catalyst IIe - According to the Invention

700 g of steatite (magnesium silicate) rings with an outer diameter of 8 mm, a length of 6 mm and a wall thickness of 1.5 mm were heated to 160 ° C. in a coating drum and mixed with a suspension of 400.0 g of anatase a BET surface area of 20 m ² / g, 57.6 g vanadyl oxalate (= 33.8 g V ₂ O ₅ ), 14.4 g antimony trioxide, 2.5 g ammonium dihydrogen phosphate (= 0.67 g P), 2.44 g Sprayed cesium sulfate (= 1.79 g Cs), 618 g water and 128 g formamide until the weight of the layer applied was 5.3% of the total weight of the finished catalyst. These precoated rings were then sprayed with a suspension of 400.0 g of anatase with a BET surface area of 20 m ² / g, 30.7 g of vanadyl oxalate, 2.45 g of cesium sulfate, 618 g of water and 128 g of formamide until the weight the applied layer was 10.6% of the total weight of the finished catalyst.

Example 10: Preparation of catalyst III - not according to the invention

700 g of steatite (magnesium silicate) rings with an outer diameter of 8 mm, a length of 6 mm and a wall thickness of 1.5 mm were heated to 160 ° C. in a coating drum and mixed with a suspension of 400.0 g of anatase a BET surface area of 20 m ² / g, 57.6 g of vanadyl oxalate, 14.4 g of antimony trioxide, 2.5 g of ammonium dihydrogen phosphate, 0.61 g of cesium sulfate, 618 g of water and 128 g of formamide were sprayed until the weight of the applied layer 10.5% of the total weight of the finished catalyst was. The catalytically active composition applied in this way, that is to say the catalyst shell, consisted of 0.15% by weight of phosphorus (calculated as P), 7.5% by weight of vanadium (calculated as V ₂ O ₅ ),
3.2% by weight of antimony (calculated as Sb ₂ O ₃ ), 0.1% by weight of cesium (calculated as Cs) and 89.05% by weight of titanium dioxide.

Example 11: Production of PSA - according to the invention and comparison

1.30 m of the catalyst III and then 1.60 m in each case of the catalysts Ia-Id and IIa-IIe were placed in a 3.85 m long iron pipe with a clear width of 25.2 mm. The iron pipe was surrounded by a salt melt for temperature control. 4.0 Nm ³ air with loads of 98.5% by weight o-xylene from 0 to about 85 g / Nm ³ air was passed through the tube from top to bottom every hour. The results summarized in the following table were obtained with a loading of 75-85 g (yield means the PSA obtained in percent by weight, based on 100% o-xylene).

Claims (12)

1. A coated catalyst for the catalytic gas-phase oxidation of aromatic hydrocarbons, comprising, on an inert nonporous support, a catalytically active composition comprising, in each case based on the total amount of catalytically active composition, from 1 to 40% by weight of vanadium oxide, calculated as V₂O₅, from 60 to 99% by weight of titanium dioxide, calculated as TiO₂, up to 1% by weight of a cesium compound, calculated as Cs, up to 1% by weight of a phosphorus compound, calculated as P, and a total of greater than 0 up to 10% by weight of antimony oxide, calculated as Sb₂O₃, wherein the catalytically active composition is applied in two or more layers, where the inner layer or inner layers have an antimony oxide content of from 1 to 15% by weight and the outer layer has, in contrast, an antimony oxide content which is from 50 to 100% lower, where the amount of catalytically active composition of the inner layer or the inner layers is from 10 to 90% by weight of the total amount of catalytically active composition, and where the proportions of constituents in the catalytically active composition are chosen from the given ranges such that they add up to 100% by weight.
2. A coated catalyst as claimed in claim 1, wherein the catalytically active composition of the inner layer or the sum of the inner layers is from 30 to 70% by weight of the total amount of catalytically active composition of the catalyst.
3. A coated catalyst as claimed in claim 1, wherein the thickness of the inner layer or the sum of the thicknesses of the inner layers is from 0.02 to 0.2 mm and the thickness of the outer layer is from 0.02 to 0.2 mm.
4. A coated catalyst as claimed in claim 1, wherein the catalyst has two concentric layers of catalytically active composition, where the inner layer contains from 2 to 7% by weight of antimony oxide and the outer layer contains from 0 to 2% by weight of antimony oxide.
5. A coated catalyst as claimed in claim 1, wherein the catalyst has two concentric layers of catalytically active composition, where the inner layer contains from 5 to 10% by weight of vanadium oxide and the outer layer contains from 1 to 5% by weight of vanadium oxide.
6. A coated catalyst as claimed in claim 1, wherein the material of the inert nonporous support is steatite.
7. A coated catalyst as claimed in claim 1, wherein the titanium oxide therein is present as titanium dioxide in the anatase form and has a BET surface area of from 13 to 28 m²/g.
8. A coated catalyst as claimed in claim 1, wherein two concentric layers of catalytically active composition are applied in the form of a shell to an inert nonporous steatite support, where, apart from titanium dioxide in the anatase form having a BET surface area of from 13 to 28 m²/g, the inner layer comprises from 5 to 10% by weight of vanadium oxide, calculated as V₂O₅, and from 2 to 7% by weight of antimony oxide, calculated as Sb₂O₃, and the outer layer comprises from 1 to 5% by weight of vanadium oxide, calculated as V₂O₅, and from 0 to 2% by weight of antimony oxide, calculated as Sb₂O₃.
9. A process for producing coated catalysts as claimed in claim 1, which comprises applying two or more than two layers of the catalytically active composition in succession to an inert nonporous support by spraying.
10. A process for producing coated catalysts as claimed in claim 1, which comprises applying two or more than two layers of the catalytically active composition in succession to an inert nonporous support by coating with the binder-containing catalytically active composition in powder form.
11. The use of catalysts as claimed in claim 1 for preparing carboxylic acids and/or carboxylic anhydrides by partial oxidation of aromatic hydrocarbons.
12. A process for preparing phthalic anhydride by partial oxidation of o-xylene and/or naphthalene by means of gases comprising molecular oxygen, wherein a catalyst as claimed in any of claims 1 to 8 is used.